The following patent documents are believed to represent the current state of the art:
U.S. Pat. Nos. 5,185,815; 5,212,547; 5,243,340; 5,375,058; 5,629,691; 5,939,987; 6,064,429; 6,181,261; 6,281,806; 6,295,007; 6,486,825; 6,563,432; 6,606,035; 6,690,295 and 6,956,493;
U.S. Published Patent Application Nos. 2002/0030609; 2002/0080046; 2002/0093433; 2002/0109625 and 2002/0163461;
German Patent No. DE 101 04 950;
European Patent No. EP 1 170 715; and
Published PCT Patent Application No. WO 02/056054.